This invention relates to systems for regulating either or both of the horizontal or vertical scan width or height of a raster scanned CRT deflection system. The invention is particularly useful in conjunction with computer terminal displays and data/graphics video projectors, for example, which may employ a very wide range of non-television standard horizontal scan rates.
A standard shunt or series (also called "boost") efficiency television line output system will drastically change both the picture size as well as the CRT anode voltage if adjusted over a wide range of operating frequencies. In copending U.S. patent application Ser. No. 250,262, filed Apr. 2, 1981 and entitled Regulation of the EHT Voltage of a CRT, there is disclosed a network that overcomes the latter problem. In accordance with the instant invention there is provided a solution to the former problem.
In U.S. Pat. No. 4,414,494, issued Nov. 8, 1983, Terrance C. Schmidt, a system for overcoming the former problem is disclosed. While this prior art system functions successfully to solve the problem, it does so by using a deflection current sensing device and a feedback loop. In accordance with the instant invention there is provided a system which overcomes the former problem in a direct way without a deflection current sensor and a feedback loop.
In a resonant efficiency magnetic horizontal deflection system, the display size (width) and high voltage (EHT) vary with the horizontal scan frequency. A decrease in horizontal scan frequency causes the width and high voltage to increase, while an increase in horizontal scan frequency causes both to decrease. This also is true of linear magnetic (non-resonant) raster scan systems such as are employed in vertical deflection, e.g., amplifiers of the push-pull type or of other types suitable for the purpose, which also may be employed in the practice of the present invention.
In a display system that has to accept horizontal scan frequencies over a wide frequency range, it therefore becomes necessary to regulate the display size (width) and EHT according to the horizontal scan frequency.
The deflection power of a resonant efficiency magnetic deflection system or of a linear magnetic (non-resonant) raster scan system can be expressed as: EQU P=Kf (1)
where:
P is deflection power; PA1 K is a constant that incorporates parameters of the picture tube and deflection yoke of a given system; and PA1 f is the scan frequency. PA1 E is the power supply voltage for the deflection system (B.sup.+); and PA1 I is the current supplied to the deflection system.
As can be seen from equation (1), the deflection power is directly proportional to the scan frequency.
The deflection power also can be expressed as: EQU P=EI (2)
where:
Equating (1) and (2) EQU EI=Kf (3)
For the display size (width) and EHT to stay constant with a change in scan frequency, the peak to peak current through the deflection yoke also must stay constant, i.e., the deflection current also becomes a constant for a given system and display size.
Equation (3) can be expressed: ##EQU1##
Since K and I are systems constants, a new constant EQU .alpha.=K/I
can be defined and (4) becomes EQU E=.alpha.f (5)
From equation (5) it may be seen that a practical solution for regulating the display size and EHT for varying scan frequencies would be to change the voltage (B.sup.+) to the deflection system according to the scan frequency.
It also can be seen from equation (5) that the relationship between the voltage supplied to the deflection system and the scan frequency is linear. This means that a control signal to regulate the voltage (B.sup.+) supplied to the deflection system can be derived directly from the scan frequency, which is the essence of the present invention. No sensors are required in the deflection system, and no feedback loops need be employed.